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Homework Help: General Energy question

  1. Mar 13, 2015 #1
    Hey guys I am in intro physics in college and took physics in high school. I have a general question about work/energy. In highschool we used work= change in total energy and total energy = PE +KE +Q (Internal) which =work. In class there was a problem that read
    2.00 kg block is attached to a spring of force constant 500 N/m. The block is pulled 4.50 cm to the right of equilibrium and released from rest.

    (a) Find the speed of the block as it passes through equilibrium if the horizontal surface is frictionless.

    (b) Find the speed of the block as it passes through equilibrium (for the first time) if the coefficient of friction between block and surface is 0.350

    What I dont understand is that my teacher said KE + PE +PEs =Wf (which is work of friction) but in highschool (the equation above the friction or Q is on the other side and she says that the friction is equal to W.

    Can anyone explain!!

    Or simply do the problem out for me I am getting mixed messages!


    first post btw
  2. jcsd
  3. Mar 13, 2015 #2

    Suraj M

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    Gold Member

    Give it a try yourself, first!
    What is this?
    Please rephrase, what do you mean by friction = W ?

    Hint: Just equate the potential energy stored in the spring to the sum of kinetic energy gained by the object and work done by friction.
  4. Mar 13, 2015 #3


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    You could write it as ##W_f = - \Delta(KE+PE)##. That is, the work done against friction is equal to the loss in mechanical energy. (You have a PEs, which I take to be another form of PE, e.g. you may have gravitational PE as well as spring PE. I'm lumping all the PEs together.). However, if your teacher takes Wf to be the work done by friction then that reverses the sign. So it could be just a question of standpoint.
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